In addition to their role as protein monomeric units, amino acids are energy metabolites and precursors of many biologically important nitrogen-containing compounds, such as heme, physiologically active amines, glutathione, other amino acids, nucleotides, and nucleotide coenzymes. Excess dietary amino acids are neither stored for future use nor excreted. Rather they are converted to common metabolic intermediates such as pyruvate, oxaloacetate, and alpha-ketoglutarate. Consequently, amino acids are also precursors of glucose, fatty acids, and ketone bodies and are therefore metabolic fuels. The degradation of amino acids converts them to citric acid cycle intermediates or their precursors so that they can be metabolized to CO.sub.2 and water or used in gluconeogenesis. Oxidative breakdown of amino acids typically accounts for 10 to 15% of the metabolic energy generated by animals.
The enzymes included in this application are involved in catabolism of the aromatic amino acids. The first reaction in phenylalanine degradation is its hydroxylation to tyrosine; thus a single pathway is responsible for the breakdown of both of these amino acids. 3,4-Dehydroxyphenyl acetate 2,3-dioxygenase is also called homogentisate 1,2-dioxygenase (EC 1.13.11.15) and, in the presence of oxygen, catalyzes the decyclization of homogentisic acid (3,4-dihydroxyphenylacetate) into 2-Hydroxy-5-carboxymethylmuconate semialdehyde. Loss of homogentisate 1,2 dioxygenase (HGO) activity is responsible for the human metabolic disorder alkaptonuria. A large number of variant forms of the human enzyme have been described which show the clinical effect of single nucleotide changes on the activity of the enzyme (Fernandez-Canon, J. M. et al. (1996). Nat Genet 14:19-24). The gene Aspergillus nidulans homogentisate 1,2 dioxygenase has been characterized and its disruption shown to induce secretion of homogenistate ((1995) J. Biol Chem 270:21199-21205). In the same article, the authors searched the GenBank database with the homogentisate 1,2 dioxygenase sequence and identified ESTs with significant similarity to homogentisate 1,2 dioxygenase. These ESTs were from tissues obtained from human, Arabidopsis thaliana, and Ricinus communis.
Fumarylacetoacetase, also named fumarylacetoacetate hydrolase (EC 3.7.1.2) catalyzes the last step in the phenylalanine/tyrosine degradation catalyzing the conversion of 4-fumarylacetoacetate and water to acetoacetate and fumarate. Debilitating mutations in this enzyme have been shown to be the cause of hereditary tyrosinemia type I in humans (St-Louis, M. and Tanguay, R. M. (1997) Hum. Mut. 9:291-299; Labelle, Y. et al. (1993) Hum. Mol. Genet. 2:941-946).